1. Field of the Invention
This invention relates to in situ recovery of shale oil; and more particularly to a two-pass method of developing a system of in situ oil shale retorts, in which retorts developed in a second pass interleaved with retorts in a first pass are isolated from toxic off gas produced from retorts developed in the first pass.
2. Description of the Prior Art
The presence of large deposits of oil shale in the semi-arid high plateau region of the Western United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits have been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; 4,043,598; and 4,192,554, which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded for forming a stationary fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the fragmented mass and advancing the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous material to produce heat, combustion gas and combusted oil shale. By continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
The off gas can contain nitrogen, hydrogen, carbon monoxide, carbon dioxide, water vapor, methane, and other hydrocarbons and sulfur compounds such as hydrogen sulfide. Carbon monoxide and hydrogen sulfide contained in the off gas are extremely toxic.
Off gas can be withdrawn from the bottom of active retorts and conveyed to a gas collection drift system. Such a gas collection drift system is disclosed in U.S. Pat. No. 4,140,343, which is incorporated herein by reference.
The gas collection drift system can be dedicated to collecting off gas from active retorts, i.e., the gas collection drifts can be isolated or sealed off to avoid leakage of off gas into adjacent areas where personnel are working.
It is desirable from an economic standpoint to excavate and develop in situ retorts in one region of an oil shale tract while retorting operations are carried on in another region of the oil shale tract. This is more economical than completing the entire system of retorts in the tract before retorting has started. However, precautions must be taken to ensure that workers in the retort development regions are not exposed to toxic off gas from active retorts in the production region.
It is desirable to maximize the amount of oil shale subjected to retorting within a region of formation being developed. It is also important to avoid subsidence in a tract of in situ oil shale retorts. There is a trade-off between retorting as much oil shale as possible to miximize resource recovery, and leaving sufficient unrecovered oil shale in the supporting pillars of unfragmented formation for supporting the weight of the overburden to avoid subsidence. Subsidence can result in fracturing of overburden with consequent leakage of water from overlying aquifiers into retort or mining areas, leakage of gas from completed retorts, leakage of air into retorts during retorting operations, and safety hazards in underground workings containing operating personnel. Such subsidence can occur when the extraction ratio in the tract is large and the remaining unfragmented formation is not sufficient for supporting the weight of the overburden.
In addition to increased stability against subsidence, a large intervening barrier pillar provides a greater resistance to air or gas leakage to or from a retort along naturally occurring (pre-existing) channels of higher permeability because of the greater distance to adjacent workings (i.e., the same principle as a labyrinth bearing seal), and the greater possibility of termination of a permeable channel in the case of discontinuous channels. In addition, the large intervening barrier pillar provides a greater barrier to heat flow from active or spent retorts into adjacent workings from damage attributed to seismic waves generated from blasting for forming a fragmented mass.
In order to maximize resource recovery while avoiding substantial subsidence of overburden, it has been proposed to develop a tract of in situ oil shale retorts by forming the retorts in rows, with the retorts in each row being relatively closely spaced and with substantial barrier pillars between adjacent rows of retorts. Such an arrangement provides support for overlying formation and protects underground workings, but the proportion of total shale oil resource recovered is diminished by the proportion of unretorted oil shale left in the barrier pillars. However, it has been proposed that additional in situ oil shale retorts be formed in the barrier pillars after initial retorting operations have been completed. Such a method of developing the oil shale tract can be referred to as a two-pass system, and an example of such a two-pass system is described in U.S. Pat. No. 4,219,237 to Sisemore.
In a two-pass system of developing in situ oil shale retorts, arrangements must be provided for subsequently retorting the barrier pillars without exposing workers to hazardous off gas.
Complete development of a tract with in situ oil shale retorts can take 40 years or more. It is desirable to commence formation and operation of retorts in the barrier pillars as soon as it is safe to do so, instead of waiting until the entire tract is developed during the first pass of retort development operations. This provides a convenient way of increasing the rate of production with limited additional capital investment.